In a magnetic recording medium for use with a floppy disk drive, there has been hitherto used, as the base film, a biaxially oriented polyethylene terephthalate film whose thermal expansion coefficient and humid expansion coefficient are relatively small, for preventing a tracking error. Even though a floppy disk has such a base film, the magnetic recording medium undergoes a dimensional change when used under high-temperature and high-humidity conditions. As a result, a magnetic head and a recorded track deviate from each other, and a tracking error occurs. At the worst case, the recording medium is curled or distorted and is hardly kept in uniform contact with the magnetic recording head. As a consequence, the coercive force and reproduction output may decrease and the recording medium may be extremely worn away. Furthermore, not only when the recording medium undergoes permanent distortion, but also when the recording medium undergoes a reversible dimensional change due to change in the environmental conditions, a track recorded on the recording medium surface and the magnetic head deviate from each other, and a tracking error occurs. For example, when the recording medium is used at a high temperature of approximately 40.degree. to 50.degree. C. and/or at a relative humidity as high as about 80%, a tracking error occurs. In particular, there is a defect in that a floppy disk on which recording has been effected at a low temperature (about 10.degree. C.) or at a low humidity (about 20% RH) causes a tracking error when reproduction is carried out in atmosphere at room temperature (about 25.degree. C.) and at a general humidity (about 60% RH). Due to this tracking error, the output envelope decreases, and the S/N ratio becomes inferior. Further, with an advance in the magnetic recording density, the thickness of the magnetic binder layer decreases, and with the increasing use of a thin magnetic metal film, the thickness of the magnetic layer decreases. As a result, the surface property, flatness and thickness unevenness of the base film increasingly exert an influence on the quality of the magnetic recording medium. When these are improper, unevenness occurs on the magnetic layer or magnetic defect occurs at a step of applying or vapor-depositing a magnetic binder, and the output decreases at a reproduction time.
Further, in a recently developed floppy disk drive used with a high-density recording floppy disk having a high recording capacity of at least 4 megabytes (MB), the magnetic layer has an increased coercive force and the number of rotation of the magnetic recording medium is increased. Therefore, the drive has a high-temperature atmosphere due to friction heat, and the magnetic recording medium vibrates greatly. Therefore, the magnetic recording medium is required to have dimensional stability and stiffness at high temperatures as compared with a conventional magnetic recording medium for a floppy disk. When a biaxially oriented polyethylene terephthalate film is used as a substrate, a track recorded on the magnetic recording medium surface and a magnetic head deviate from each other due to a dimensional change caused at a high temperature since it is poor in heat resistance, and a tracking error occurs. Further, with an increase in the number of the rotation, the floppy disk causes fluttering and fails to keep contact with a magnetic head under given conditions, and a recording error or a reproduction error occurs.